1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst that purifies carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine of a gasoline or diesel automobile, a boiler or the like. More particularly, the present invention relates to an exhaust gas purifying catalyst that purifies a large amount of HC discharged in a low temperature range at time of starting up an engine, in which a three-way catalyst is not activated.
2. Description of the Related Art
Heretofore, in order to purify exhaust gas from an internal combustion engine of an automobile or the like, a three-way catalyst that simultaneously performs oxidation for carbon monoxide (CO) and hydrocarbons (HC) and reduction for nitrogen oxides (NOx) has widely been used. However, since the temperature of the exhaust gas is low and the three-way catalyst disposed on an exhaust gas passage does not reach an activation temperature immediately after the start-up of the engine, a large amount of cold HC discharged in this case cannot be purified.
Recent years, for the purpose of purifying such cold HC, an HC-adsorbing/purifying catalyst (HC-trap catalyst) as a three-way catalyst having an HC adsorbing function has been developed, which includes a hydrocarbon adsorbent (HC adsorbent) and a purifying catalyst such as a three-way catalyst.
The HC-trap catalyst temporarily adsorbs and holds cold HC discharged in the low temperature range at the time of starting up the engine, in which the three-way catalyst is not activated. Then, the HC-trap catalyst gradually desorbs the HC and purifies the desorbed HC by the purifying catalyst when the three-way catalyst is activated due to a temperature increase of the exhaust gas.
As the HC adsorbent, zeolite is generally used. As the catalyst purifying the HC desorbed from the HC adsorbent, a catalyst obtained by mixing noble metal species such as rhodium (Rh), platinum (Pt) and palladium (Pd) on the same layer and a catalyst of a multilayer structure including Rh and Pd layers have been proposed.
Japanese Patent Laid-Open Publication H2-56247 (published in 1990) discloses an exhaust gas purifying catalyst including a first layer mainly containing zeolite and a second layer provided on the first layer. The second layer mainly contains noble metals such as Pt, Pd and Rh.
Other HC-trap catalysts have been disclosed in Japanese Patent Laid-Open Publications H6-74019 (published in 1994), H7-144119 (published in 1995), H6-142457 (published in 1994), H5-59942 (published in 1993), H7-102957 (published in 1995), H7-96183 (published in 1995) and H11-210451 (published in 1999).